Mercaptonitriles for the removal of tarnish on metals

ABSTRACT

MERCAPTONITRILES ARE EFFECTIVE FOR THE REMOVAL OF CORROSION PRODUCTS SUCH AS TARNISH FROM METALS OTHER THAN FERRO AND ALUMINO METALS, SUCH AS COPPER, SILVER, TIN, LEAD, ZINC, CADMIUM, AND ALLOYS THEREOF.

United States Patent Oflice- 3,707,507 Patented Dec. 26, 1972 ABSTRACT OF THE DISCLOSURE Mercaptonitriles are effective for the removal of corrosion products such as tarnish from metals other than ferro and alumino metals, such as copper, silver, tin, lead, zinc, cadmium, and alloys thereof.

This invention relates to the treatment of metal surfaces. In another aspect, it relates to the removal of tarnish-causing substances from metal surfaces.

Tarnishing of the surfaces of many metals has been a problem for centuries. Metal surfaces, either of pure metals or many alloys, when freshly prepared and clean have an attractive bright shiny appearance. However, many metal surfaces tend to become dull, to lose their luster, to develop unattractive discolorations, due to gradual reaction of the surface with oxygen in the air, or frequently with various and sundry sulfur compounds in the air present in ever increasing abundance due to environmental factors.

Cleaning of the surfaces generally has been with various grinding or polishing compounds containing abrasives of various nature and types, frequently mainly abrasives and a liquid, sometimes with chemical additives which, however, have not necessarily assisted the cleaning process.

The use of abrasives, generally, is undesirable. The metal surface is worn away to some extent, however slight, at every abrading. In time this results in considerable wear and tear, particularly with plated objects. Further, the abrading itself, while removing the tarnish or corrosion, and so exposing a fresh surface, at the same time produces a myriad of minute scratches, thus actually increasing the surface area of metal exposed to the tarnishing atmosphere, and so usually results in increased tarnishing rates.

An effective class of cleaning agents is needed, that will remove discolorations, surface oxidation or sulfurization, tarnish, and the like from a number of metal surfaces, with minimal undesirable effects to the metal surface.

Therefore, it is an object of my invention to provide chemical cleaning agents to clean metal surfaces without the use of an abrasive.

Other aspects, objects, and the several advantages of my invention will be apparent to one skilled in the art from the following description and appended claims.

I have discovered that the class of compounds known as rnercaptonitriles are effective agents for the removal of tarnish from the non-ferro and non-alumino metals and alloys in general. I have found the rnercaptonitriles to be particularly efiective for such as copper, silver, tin, lead, zinc, cadmium, and the various and sundry alloys thereof. By alloys, I refer to those solid mixtures of two or more metals, or of one or more metals with various non-metallic elements, egnerally prepared by fusing of the components. I include those alloys containing tarnishable metals in the alloy such as to include one or more tarnishable metals, or two or more together such as copper plus tin, or copper plus zinc.

The rnercaptonitriles useful in accordance with my invention can be represented by the general formula:

The total number of carbon atoms per individual mercaptonitrile compound can range widely, though a minimum of two including the carbon in the nitrile group is required. Preferably the range is from 2 to 12 carbon atoms per molecule, with the upper limit primarily dictated by upper feasible solubility limits of the mercaptonitrile compound in applications according to my invention. Each R group in the formula is individually selected from hydrogen, alkyl, cycloalkyl, aryl, and aralky. The integer indicator in the formula, n, can be from 0 to 4 inclusive.

Examples of suitable rnercaptonitriles include a presently preferred species 3-rnercaptopropionitrile, as well as 2-methyl-3-mercaptopropionitrile, 2-benzyl-3-mercaptovaleronitrile, 2-methyl-3-mercaptovaleronitrile, 3-cyclohexyl-3-mercaptopropionitrile, 6 mercaptohexanenitrile, Lmercaptoacetonitrile, Z-mercaptopropionitrile, 3-benzyl- 2-mercaptovaleronitrile, 3 phenyl 2 mercaptobutyronitrile, and the like. Of course, two or more rnercaptonitriles can be used together in admixture.

The rnercaptonitriles can be prepared by any method of preparation known to the art. One method of preparing rnercaptonitriles, such as the presently preferred 3-mercaptopropionitrile, is disclosed in U.S. Letters Patent 3,280,164, Rector P. Louthan, issued Oct. 18, 1966, and/ or by free radical addition of hydrogen sulfide to the double bond of unsaturated nitriles such as described in U.S. Letters Patent 3,278,526, Rector P. Louthan and R. L. Cobb, issued Oct. 11, 1966.

In the process of my invention, the entire metal object itself where it is of a convenient size, or otherwise a tarnished surface of a metal or alloy or a metal coating or sheeting on some object or surface, is contacted by a primarily aqueous solution or dispersion of the mercaptonitrile or mixture of rnercaptonitriles.

A solution or dispersion of the mercaptonitrile is used for purposes of economy, since it is unnecessary to use large quantities of the mercaptonitrile itself for the cleaning of a tarnished metal surface. The cleaning solutiondispersion can be brushed on the object or surface, or sprayed onto the tarnished 0r discolored metallic surface such as by pressurized sprayer or aerosol, or the object itself can be wholly or partly dipped into the cleaning formulation. After exposure to the cleaning formulation, the metal surface usually is rinsed with water and wiped dry. For badly discolored surfaces, some mild pressure with a cleaning cloth can be applied to assist in brightening the surface.

The cleaning solution-dispersion usually will contain from about 0.005 to 20 weight percent or more, weight of mercaptonitrile content relative to the total weight of the cleaning solution, with a preferred range of from about 0.05 to 10 weight percent, and a presently most preferred content of the order of about fweight percent. The optimum range for mercaptonitrile ingredient content in the cleaning solution-dispersion depends, at least in part, on the solubility of the particular species chosen, and the nature of the metal tarnish being removed. Dispersions or solutions of relatively strong strength can be prepared, of course, and diluted further at time of usage, where desired or found convenient to prepare a concentrated solution for bulk storage and then dilute as needed for actual application.

While the aqueous media is presently preferred and convenient, it is also quite feasible to use, along with the water, varying proportions of one or more alcohols, such as up to 30 weight percent of water plus alcohol, particularly of the lower alcohols of 1 to 4 carbon atoms per molecule, including methanol, ethanol, either of the propanols, or any of the butanols. The alcohols particularly are useful in assisting in wetting a metal surface to be cleaned so that the mercaptonitrile itself can be most effective.

Generally, a stabilizing agent, particularly an antioxidant, is added to the formulation in order to increase storage or shelf life, assisting in avoiding oxidation of the mercaptonitrile content. The cleaning solution formulation incorporating the selected mercaptonitrile or mercaptonitriles preferably is prepared so as to contain a stabilizing additive selected from the ammonium or alkali metal hyposulfites or hydrosulfites (dithionites), sulfites including the hydrogen sulfite or bisulfite, and pyrosulfate or metabisulfite. The alkali metals, of course, include lithium, sodium, potassium, rubidium, and cesium. Shelf life studies of mercaptonitrile cleaning solutions incorporating a 2:2:96 composition of S-mercaptopropionitrile: anhydrous sodium sulfite:water indicated no loss in effective activity after three weeks, and only very slight loss in effectiveness even after 12 weeks.

The sulfite or other sulfur-based stabilizer as described above can be used alone, or in admixture as two or more thereof, and as either the anhydrous or hydrated form. The stabilizer usually is used in the range of from 1:10 to 5:1, preferably approximately 1:1, weight ratio of stabilizer to the mercaptonitrile content of the cleaning solution formulation. The presently preferred stabilizer is sodium sulfite for reasons of economy and availability.

The mercaptonitrile cleaning formulation preferably is prepared with an alkaline balance for most effective re moval of most metal tarnishes and discolorations. The alkaline range of a pH of between about 8 and 12 is preferred. The pH of the mercaptonitrile cleaning formulation can be adjusted to the desired pH range in any conventional manner, including the use of ammonia, or of ammonium or an alkali metal hydroxide, or of an organic amine, or combination thereof, in such amount or amounts as are sufficient to provide or to assist in providing the desired pH area range. Examples of suitable amines that can be used are those containing from about 1 to 12 carbon atoms per amine, and include pyridine, morpholine, ethylamine, ethanolamine, di-n-propylamine, tributylamine, trimethylamine, diethanolamine, and the like.

Other alkaline substances can be utilized, such as the alkali metal carbonates, particularly where a moderately alkaline pH range is desired. Any of the ammonium or alkali metal phosphates can also be utilized, particularly those which have an alkaline balance, i.e., hydrolyze in water, to provide suitable pH range, or to assist in providing a suitably buffered pH range. Some of these phosphates, such as hexametaphosphates, trimetaphosphates, pyrophosphates, tripolyphosphates, and the like, additionally are useful in imparting properties to the cleaning solution to assist in metal surface treatment, such as assisting in removing dirt and the like, so that the mercaptonitrile can most effectively operate against the tarnish and discoloration.

When the stabilizer itself is slightly alkaline, the pH of the cleaning solution incorporating the mercaptonitrile often can be wholly or partly adjusted by the resulting hydr lysis Of the chosen stabilizer in solution. For example, the alkali metal sulfites, such as sodium sulfite, used in aqueous media along with the mercaptonitrile, frequently produce a pH in the neighborhood of approximately 8.5, which provides a composition suitable for removal of many metal tarnishes. If a higher pH is desired, then additional alkaline material, such as a hydroxide, carbonate, phosphate, amine, and the like, can be added to the formulation.

When it is desired to prepare concentrated solutions of mercaptonitrile, a higher pH is of assistance, and an emulsifier frequently is desired. With higher molecular weight mercaptonitriles, an emulsifier frequently is required. Where an emulsifier is used, a typical formulation includes a weight ratio of mercaptonitrile:emulsifier of about 10:1, with a broad range of from 5:1 to :1, depending on the molecular weight of the mercaptonitrile, the concentration of mercaptonitrile desired, and the pH range used, since higher pH values also assist in preparing more concentrated dispersions of mercaptonitrile. Various suitable emulsifiers can be determined from references such as Detergents and 'Emulsifiers-Up to Date, John W. McCutcheon, Inc. (1962), and bulletins of the United States Department of Agriculture, Bureau of Entymology and Plant Quarantine, such as Bulletin E-607. Presently preferred emulsifiers are the non-ionic types such as the ethoxylated alkylphenols.

The cleaning formulation can incorporate other ingre dients frequently used in metal cleaning compositions, such as wetting agents, detergents, surface active agents, colorants, odorants, odor masking components, and the like, all of known type and usage to those skilled in the metal cleaning arts. Various components, such as the alcohols, amines, emulsifiers, serve double duties other than the purposes for which we have described, since they assist in cleaning surfaces contaminated with dirt, grease, oily matter, and the like, which might otherwise interfere in the activity of the tarnish removing formulation itself. Surfaces containing considerable amounts of dirt, grease, oily matter, and the like, can be treated for dirt and oil removal, such as by washing with a detergent, before treating according to my invention.

To further illustrate my invention, tests were made on various metal strips. The materials used in the tests should be considered illustrative, and not as limitative as to the particular components used, or of the metals from which tarnish can be removed.

EXAMPLE A metal cleaning solution according to my invention was prepared by dissolving 2.0 g. (gram) of 3-mercaptopropionitrile in a mixture of 2.0 g. of anhydrous sodium sulfite and 96 g. of water. The admixture was stirred at room temperature, becoming homogeneous, and thus was then ready for use.

Badly tarnishd pure copper strips and sterling silver strips were immersed for approximately 10 seconds per strip in the cleaning solution prepared as above.

This brief immersion rendered the strips bright and shiny after the simple immersion treatment. The strips were removed, rinsed with water, and remained bright for a considerable time.

Similarly, various samples of tarnished, discolored lead, zinc, and cadmium, also were treated, and similarly cleaned, easily, simply, effectively.

These tests with a series of different metals clearly show the effectiveness of my mercaptonitriles as cleaning formulations to remove tranish and discoloration from a variety of metals, neatly, efficiently, and effectively.

Certainly, reasonable variations and modifications are possible within the scope of my disclosure, yet without departing from the reasonable scope and certainly not from the intended spirit thereof, as shown both by the specification itself and including the claims here appended.

I claim:

1. A process for the removal of tarnish from non-ferro and non-alumino metal surfaces which comprises bringing said tarnished metal surface into contact with at least one mercaptonitrile, wherein said mercaptonitrile contains at least 2 carbon atoms per molecule and is represented by AH 1'1 wherein each R is individually selected from hydrogen, alkyl, cycloalkyl, aryl, and aralkyl, and n is an integer of from to 4 inclusive.

2. The process according to claim 1 wherein the total number of carbon atoms per individual mercaptonitrile compound is from 2 to 12 inclusive.

3. The process according to claim 2 wherein said metal is copper, silver, tin, lead, zinc, cadmium, or combination or alloy thereof.

4. The process according to claim 3 wherein said mercaptonitrile comprises a solution-dispersion of mercaptonitrile in water, and the solution-dispersion contains about 0.005 to 20 weight percent of mercaptonitrile based on the total Weight of solution-dispersion.

5. The process according to claim 4 wherein said solution-dispersion further contains lower monohydric alcohol, stabilizing agent, alkaline material, emulsifier, or combination,

wherein said stabilizing agent is ammonium or alkali metal hyposulfite, sulfite, or meta-bisulfite, said alkali metal is lithium, sodium, potassium, rubidium, or cesium; and said alkaline material is ammonia or ammonium or alkali metal hydroxide, phosphate, or carbonate, or organic amine.

6. The process according to claim 5 wherein said lower monohydric alcohol contains from 1 to 4 carbon atoms;

said stabilizing agent is present in said solution-dispersion in the range of 1:10 to 5:1 based on the weight ratio of stabilizing agent to mercaptonitrile,

and said alkaline material is present in an amount sufficient to adjust the pH of said solution-dispersion to within the range of 8 to 12.

7. The process according to claim 6 wherein said solution-dispersion further contains at least one said emulsifier in a concentration of from 5 :1 to :1 weight ratio of mercaptonitrile:emulsifier.

8. A metal cleaning formulation effective for cleaning copper, silver, tin, lead, zinc, cadmium, or combinations thereof, which comprises at least one mercaptonitrile, water, and stabilizing agent,

wherein said mercaptonitrile is represented by wherein each R is individually selected from hydrogen, alkyl, cycloalkyl, aryl, and aralkyl, and n is an integer of from 0 to 4 inclusive, such that the total carbon atoms per mercaptonitrile ranges from 2 to 12; and said mercaptonitrile is present in a concentration of 0.005 to 20 weight percent based on total weight of said cleaning solution-dispersion.

9. A metal cleaning solution-dispersion according to claim 8 wherein said solution-dispersion is adjusted with alkaline material to a pH of between 8 and 12, and said alkaline material is ammonia, ammonium or alkali metal hydroxide or phosphate, alkaline organic amine of up to about 12 carbon atoms, or combination thereof.

10. The metal cleaning solution-dispersion as described in claim 9 further incorporating from 5 :1 to 100:1 of at least one emulsifier based on the weight ratio of mercaptonitrilezemulsifier.

References Cited UNITED STATES PATENTS 3,278,574 l0/ 1966 Louthan 260465.1 3,280,164 10/1966 Louthan 260465.l 3,502,708 3/1970 Thoma et al 260465.1 3,248,235 4/ 1966 Pryor et al. 106-3 MAYER WEINBLATI, Primary Examiner U.S. Cl. X.R. 

